NDVI relative in PNG (all NDVI value in whole ramp color)

[tecnologia (Tecnologia ICrop)]

I want to create a function that put any NDVI data range in all ramp color, doesn’t matter the interval of the values. In other hands, I’m trying to create a NDVI relative when all ramp color represent the value range. I’m just tried the blend method in utility functions, but did not work as I expected. After, I used my own logic to build a way to do this as follow in the function:

function evaluatePixel(sample) {
  if ([3, 8, 9, 10].includes(sample.SCL) ){
    return [1, 0, 0]
  } else {
    let ndvi = (sample.B08 - sample.B04) / (sample.B08 + sample.B04);
    let min = Math.min.apply(ndvi);
    let max = Math.max.apply(ndvi); 
    let ndviRelative = (ndvi - min) / (max - min);
    return [...visualizer.process(ndviRelative), sample.dataMask] 

Ignoring the dataMask and SCL samples, note that I’m trying to use “Math.min/max.apply()” method to extract de min and max values from ndvi and use this in the formula that’s normalize any NDVI data between 0 and 1 to then, vizualize in a ramp color. However, the return it’s been an image with a single data (in attach). My partiners suspect that I have to use updateOutputMetadata instead evaluatePixel. So, what should I have to do?

[chung.horng (Chung)]

Hi @tecnologia ,

It seems to me that you’re trying to obtain the maximum and minimum NDVI value of your AOI and use the NDVI range to create a visualisation. If this is the case, this can’t be achieved in one request as Processing API is pixel-based and the maximum/minimum is unknown until the execution completes.

To create a visualisation based on the NDVI range of your AOI, you will have to make a Statistical API request first to get the maximum and minimum, then apply them to your Evalscript.

[tecnologia (Tecnologia ICrop)]

Thanks for the answer @chung.horng. It’s exactly what I want.

So, I need to make Statistical API for the max/min NDVI data and then, the process API, right?

[chung.horng (Chung)]

Yes, exactly. To parse a variable (the min/max in your case) to Evalscript, please you can have a look at cell 10 in this example.